Linear tomography is a classic imaging technique (dating from the 1930s) used to create medical x-rays which are in focus at a single plane within a patient, but out of focus everywhere else. This approach may reduce confusion due to overlying (superimposed) anatomical structures, thereby improving the diagnostic utility of the x-ray. Modern versions of linear tomography using digital x-ray images (as opposed to film images) are known as tomosynthesis. A discussion of tomography and tomosynthesis is given in the article “Digital computed laminography and tomosynthesis—functional principles and industrial applications” by S. Gondrom et al., NDT.net, July 1999, Vol. 4 No. 7, parts of which are paraphrased and summarized in the background section herein.
X-ray irradiation is well known as a non-destructive testing method for technical components. Unfortunately, using simple irradiation techniques, there is no possibility to get information about the depth of the imaged structures. In 1932 de Plantes performed first experiments to image an object layer by layer. The technique was called laminography and was used in medical diagnostics. Furthermore the development of computed tomography (CT) allowed a nondestructive imaging of object slices, but with the restriction that the objects have to be irradiated from the full angular region. Because of high absorption and limited access, this is not always possible, e.g. in the case of flat components as multilayer printed circuit boards or welding seams in big components.
Laminographic methods are able to overcome these difficulties. They yield images of object slices and allow the determination of the position of the object structures.
Classical laminography is based on a relative motion of the x-ray source, the detector and the object. The x-ray source and the detector are either moved synchronously on circles or are simply translated in opposite directions. Due to that correlated motion, the location of the projected images of points within the object moves also. Only points from a particular slice, the so called focal slice, are projected always at the same location onto the detector and therefore imaged sharply. Object structures above and below the focal slice are projected at different locations. Because of that, they aren't imaged sharply and are superimposed as a background intensity to the focal slice. This principle of superimposing projections is called tomosynthesis.
Of course rotational laminography needs a more complicated mechanical scanning system than translational laminography, however, it yields better results, because of the bigger angular region from which projections are obtained.
The main disadvantages of classical laminography are the background intensity that reduces the contrast resolution, the complicated mechanical scanning system and the fact that, in each measurement, only one slice is imaged sharply. All other slices have to be inspected consecutively by displacing the object vertically.
The only difference between digital and classical laminography is the use of a digital x-ray detector so that a series of discrete projections may be digitally stored. Nevertheless, this helps to overcome some of the above mentioned disadvantages. For example, all object layers may be obtained with only one measurement by sorting the data. Therefore, it becomes possible to test objects 3-dimensionally within acceptable times. Moreover, there is the possibility to reconstruct the projection data measured under many angles using well known CT reconstruction algorithms like the Algebraic Reconstruction Technique (ART). This leads to a higher contrast resolution and overcomes the smearing out effect of simple tomosynthesis, but needs more time.
The Fraunhofer Institute Nondestructive Testing IZFP developed a laminographic method, named computed laminography (CL), which only requires a simple linear translation of the object through the fan beam of an x-ray source. Both the x-ray source and the detector remain stationary. Alternatively, the object may remain stationary and the x-ray source and the detector may be moved synchronously but without a relative movement. Therefore, it becomes very simple to examine e.g. large and heavy objects that normally cannot be easily examined with classical laminography because of the complicated mechanical system set-up.
During the movement, the object is irradiated by the x-rays under different angles due to the fan beam with an opening angle θ. Therefore, the elements of the detector get successive information of a given volume element of the object under consecutively changing angles and these digital projections contain the complete structure information of all object slices. To obtain cross sections comparable to classical laminography, the projection values simply have to be sorted and added correctly.
CL is equivalent to a CT with a limited angular region, allowing the use of special CT reconstruction algorithms like ART to enhance contrast resolution. Additionally, it is possible to integrate ‘a priori’ information to these algorithms reducing the reconstruction time and the artifacts caused by the limited angular region and leading to a higher image quality.
Artifacts arise because projections are only obtained from a small aperture. This reduces furthermore the geometrical resolution compared with a traditional CT. There is a dependence of the ratio of the axial to lateral resolution as a function of the fan beam opening angle. The axial resolution, that is in line with the x-rays, is always smaller than the lateral resolution, perpendicular to this direction.
Besides the several possibilities in acquiring data with laminographic methods, large differences may be achieved in the quality of reconstructed cross sections by varying the reconstruction method. A simple tomosynthesis yields results in a relatively short time, but normally the cross sections are blurred and only few details with high contrast are visible. Using, for example, ART, which is an iterative reconstruction technique, more time is needed but more details may be seen.
Digital laminography is a suitable method to examine flat components like printed circuit boards or welding seams. For the examination of printed circuit boards, there even exist several industrial systems like e.g. the Feinfocus μ-3D Visualiser or the HP 5DX Series II, formerly known as the Four Pi System.
Laminographic methods turn out to be excellent x-ray methods for the inspection of flat components like printed circuit boards or welding seams in big and flat components. Compared with classical laminography, the use of digital x-ray detectors has a lot of advantages and makes it possible to use digital laminography as a modern industrial NDT method.
It may be possible to apply modified versions of such techniques using visible light spectrum imaging devices as well.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such systems and methods with embodiments of the present invention as set forth in the remainder of the present application with reference to the drawings.